In this paper the role of hyperfine interactions in molecular ortho-para conversion is considered. The main experimental results in the CH₃F spin conversion are briefly described. The general theory of nuclear spin conversion in molecules based on intramolecular ortho-para state mixing is given. Two types of ortho-para state mixing are taken into account: the spin-spin interaction between molecular nuclei and the spin-rotation interaction. The level-crossing spectra for the spin conversion in ¹³CH₃F and ¹²CH₃F molecules are calculated for an electric field range of 0 - 10⁴ V/cm. The spin conversion spectrum, discussed here, is an example of a new type of Doppler-free spectroscopy.

For the purpose of combustion diagnostics using hydrogen as a probe molecule, the usual perturbing molecules are O₂, N₂, H₂O, etc. For such molecular systems at high temperature, only semiclassical calculations of line broadening parameters are possible. Bykov et al. (Atmos. Oceanic Opt., 5, 587, 1992; ibid. 5, 728, 1992) have recently proposed improvements of a model previously developed by two of the authors, by introducing exact trajectories. The aim of this lecture is to present the first numerical tests of such improvements for the simple H₂-He system, for which close coupling calculations are also feasible. A good consistency is obtained with experiment. The extension to molecular systems of practical interest is thus promising.

Beside the knowledge of line positions and strengths precise data on the line broadening are prerequisite for the analysis of atmospheric transmission spectra and the in-situ determination of trace gas concentrations. Moreover, experimental data on the line broadening provide information on the collision processes as origin of broadening and shift of absorption lines. In this paper result on the line broadening obtained with a pulse driven diode laser spectrometer are presented. Intensive studies of the self- broadening were carried out for the v₁, v₃, and v₁ plus v₃ bands of SO₂, the v₁, v₂, v₃, and 2v₂ bands of H₂S, and the v₃ band of NO₂ in order to analyze the quantum number dependence of the broadening coefficients. For SO₂ a pronounced decrease of the broadening coefficients with increasing quantum number K_aⁿ was found. In the case of H₂S the Jⁿ-dependence dominates, whereas in the case of NO₂ a trend is not obvious. To study the perturber dependence of the broadening, air, He, Ne, Ar, Kr, and Xe were included in the experiments. The tendencies observed for noble gas perturbers could be described theoretically with a modified Anderson-Tsao-Curnutte-theory.

The aim of the present paper is to collect the spectroscopic data assembled so far on small water polymers in the gas phase as well as on host clusters and in cryogenic matrices and to compare them with each other. In addition it is shown that the whole set of data can be regarded as highly consistent if the frequency shifts are analyzed in terms of a correlation with the square root of the critical temperature of the gas constituting the matrix or host cluster.

Microwave electro-optic modulation of a carbon-dioxide laser beam using CdTe crystals yields a widely tunable infrared source of high spectral purity. This radiation has been used to investigate subDoppler saturation spectra of various molecules like SF₆, C₂H₄, NH₃, OCS or CDF₃. Using two independently tunable electro-optic modulators, it is possible to perform infrared-infrared double resonance with subDoppler resolution. Such a scheme has been used to study the 2v₃ - v₃ hot band transitions of SF₆. The spectrometer design and the experimental details are described. The contribution of these techniques to the knowledge of the molecular parameters are discussed.

Semiempirical intermolecular diatomics-in-molecule first- order perturbation theory for the rare gas atom -- halogen molecule interaction potentials is implemented for accurate quantum calculations on the structure, vibrational energy levels, and decay dynamics of the corresponding van der Waals complexes. The results obtained for Ar. . .I₂, Ar. . .Cl₂, and Ne. . .Cl₂ complexes demonstrate the high accuracy of semiempirical potential energy surfaces.

The model based on the effective non-hermitean Hamiltonian is implemented for studying spectrum of the vibrational metastable states of the Ne. . .I₂ van der Waals complex. The non-trivial behavior of the resonance positions at high vibrational excitations of the I₂ fragment is explained by the intramolecular vibrational energy redistribution through intermediate resonance states. The interference effects in the resonance spectrum are elucidated by a comparative analysis of models with different choices of the active space in the Feshbach partitioning technique. It is shown that the real part of the optical potential (which is usually neglected) significantly affects positions of metastable states.

Our method of attack of the molecular rotation in the case when the ro-vib interaction can not be considered as a small one involves three steps. Firstly, so called effective classical energy of the molecular rotation is defined as the minimum of the exact classical ro-vib Hamilton function (it is suggested that the dependence of Hamilton function on the angular momentum components is the parametric one). The second step consists in the qualitative analysis of the classical rotational energy function using well known conception of the rotational energy surface (RES). The non- local bifurcation in rotational dynamics of isotopically substituted P₄ molecule is considered as an example. The third step consists in the construction of an effective quantum rotational Hamiltonian from the classical rotational energy function. As an example, the rotational levels of KCN molecule in the ground vibrational state are calculated.

Advantages of Hamiltonians in the principal axes of inertia which are used for studying nonrigid molecules are set forth. Representation of orbital angular momentum through non-holonomic components of rotations of dynamical axes is stated. Peculiarities of three modifications of rovibronic Hamiltonians are considered, with (1) the dynamical axes are associated with the nuclear subsystem; (2) nuclei and electrons are treated uniformly when the principal axes are brought into use; (3) a Hamiltonian of linear molecules is represented as the Hamiltonian in the principal axes. Special attention is paid to the use of non-holonomic variables.

Quantum corrections to a semiclassical approximation for an asymmetric top are derived on the basis of the semiclassical self-consistent vibration-rotation fields theory. This can be considered as a foundation for describing of highly excited vibration-rotation states of nonrigid molecules.

A new version of many-body perturbation theory (MBPT) based on the parent molecular orbitals (MOs) and the basis sets adjusted to each individual state is developed. Excitation energies computed by this MBPT are in good agreement with large-scale configuration interaction (CI) results.

The relations between spectroscopic constants of the series for the energies of vibration-rotational states of a diatomic molecule constructed in references 1 and 2 and the constants Y_ij of the Dunham series are reported. The precise experimental data on the energy levels of the molecular HCl comprising practically all the discrete vibration-rotational spectrum in the ground electronic state were treated. It has been proved that the series constructed in references 1 and 2 possesses much better convergence and predicting ability than the Dunham series and enables one to derive the Y_ij constants from the adequate treatment of the whole set of experimental data concerning to the discrete spectrum of a molecule.

In recent years there has been a great deal of interest in excited states of the water molecule with a number of experimental and theoretical studies reported. The umber of spectroscopic studies of complexes involving hydrogen bonding has greatly increased. However, most of this research has involved studies of the IR and Raman vibrational spectra of the molecule hydrogen bond formation strongly perturbs the vibrational transitions, but the changes in the electronic spectrum can also be observed. The interaction between water molecules is dominated by hydrogen bonding and the level of the ab initio theory is used because it accurately reproduces the major components of the interaction energy. Ab initio studies of cyclic water clusters (H₂O)_n, n equals 2 minus 6 and analysis of many-body interactions for ground states of these clusters has been done. The magnitudes of the two-through six-body energy terms and their contribution to the interaction energy of small ring water clusters has been computed at the Hartree-Fock (HF) and second-through fourth-order many-body perturbation (MP2, MP4) levels of theory. In this paper, we investigated the lowest S₁ excited electronic states of the cyclic water clusters (H₂O)_n, n equals 2 minus 6 from the point of changes in the absorption spectra for many-body interactions and examined their trends as the cluster grew.

The ultraviolet absorption of N₂O has been studied by many investigators. References to earlier works are given by Zelikoff, Watanabe and Inn. There is general agreement that from 168 nm upwards nitrous oxide shows weak continuous absorption which reaches a maximum variously placed at 182 and 184 nm. Above 184 nm the absorption rapidly decreases. The weak character of the absorption suggests that the transitions in this region are forbidden. Any spectral structure in this region is absent therefore it is difficult to say what electronic transitions involved here. For the detection of N₂O it is useful the knowledge of absorption coefficient N₂O at 180 - 300 nm for different temperature. The dependence of the intensity of absorption on temperature was noted by Nicolle and Vodar and studied later by Holliday and Reuben. It is found that absorption increased rapidly with temperature. A graph of the logarithm of the extinction coefficient at 230 nm against the reciprocal temperature gave almost straight line with a slope corresponding to 240 nm. The absence of any fine structure of continuum is the reason to obtain the empirical formula for the calculation of absorption coefficient N₂O at 180 - 300 nm and temperature 290 - 2000 degrees Kelvin. Our absorption coefficient K, in cm^-1, is defined by I/I₀ equals exp(minus Kx) where x is the layer thickness of nitrous oxide in cm reduced to S.T.P. To obtain the formula we used the expression for absorption coefficient at continuous profile of vibronic transition.

Abstract not available.

The spectroscopic and electro-optical exhibitions of large amplitude motions within the molecules NH₃ and L[MX₃] (L equals Li,Na; M equals Be,Mg; X equals H, F, O) are investigated. The large amplitude umbrella motions within the ammonia molecule are considered on the basis of one-dimensional dynamic calculations. The NH₃ molecule dipole moment behavior oscillating with the time is obtained. On the basis of ab initio calculated potential energy surfaces (PES's) and dipole moment functions, using the finite element method, the investigations of the spectroscopic and electro-optical exhibitions of intramolecular rotations within L([MX₃] type molecules are carried out. The tunneling splittings of energy levels with number n which are accessible for high resolution vibrational spectroscopy appear for these molecules only at n greater than or equal to 10.

To describe contribution of the parent molecule rotation and the interfragment repulsive forces into the rotational excitation of molecular fragments produced through the photo-dissociation process, a simple model is proposed. The model is proved to be useful in estimating the magnitude of the additional angular momentum, that originates from the interfragment repulsive forces, by observation of the polarized fluorescence of photoproducts.

The present paper is a continuation of our effort to analyze high resolution H₂S absorption spectra from 2000 to 11,500 cm^-1 using the Fourier-transform spectrometer at Kitt Peak. Previously, the observed band centers for 30 states were assigned and fitted in a vibrational analyses, and local mode behavior was demonstrated in four bands near 1 micrometer. Here we present the first (2(nu) ₂, (nu) ₁ and (nu) ₃ near 4 micrometer) and second 3(nu) ₂, (nu) ₁ plus (nu) ₂ and (nu) ₂ plus (nu) ₃ near 2.7 micrometer) triad system.

This work is a continuation of our theoretical analyses of the H₂S Fourier-transform spectrum, recorded in Kitt Peak National Observatory, in 2000 - 11,000 cm^-1 spectral region. This time we deal with 4,500 - 5,600 cm^-1 spectral region where the first hexad of vibrational bands (nu) ₁ plus (nu) ₃, 2(nu) ₂ plus (nu) ₃, (nu) ₁ plus 2(nu) ₂, 4(nu) ₂, 2(nu) ₁, 2(nu) ₃ are placed. In comparison with the previous work significantly more precise vibrational-rotational energy levels of the analyzed vibrational states have been obtained from the spectrum assignment, which was performed with the use of special computer program. The parallel refinement of the rotational and dipole moments parameters allowed us to make reliable predictional calculations and assign not only strong lines, included in combination differences, but a weak single lines also. Finally 1052 precise vibrational- rotational energy levels for all six members of the hexad have been derived in comparison with 542 from our previous work.

Some fittings are made to treat the spectroscopic parameters so as to adjust not only the potential energy function but also the on-adiabatic parameters of diatomic molecules. Definitions of the effective reduced mass and the effective internuclear distance in relation to the rotational constant are used for this purpose. The detailed calculations for CO molecule are given.

Fragmentation dynamics of the HeBr₂(B, (nu) ) complex is studied by mens of three-dimensional quantum calculations for a broad range of vibrational excitations ((nu) ). Using additive pairwise potentials for both electronic states involved, X and B, resonance line shapes, lifetimes, average structures, spectral shifts and final product distributions are presented for an assumed main transition (X,J equals 1, j equals odd, parity equals negative) yields (B,J equals O,j equals even, parity equals positive). For (nu) equals 44, a partial closing of the (Delta) (nu) equals negative one channel of the fragmentation is found, in good agreement with the experiment.

Highly correlated ab-initio potential curves are calculated for the fundamental states of CH, NH, OH and SH. These potentials are first used in a numerical treatment of nuclear motion then fitted with modified Morse potential functions.

Peculiarities of spontaneous soliton formation under arbitrary shape, powerful optical pulses propagation through resonantly absorbing molecular media are studied. The resonance transition takes place within vibration-rotation band. Nonresonant absorption by the rest medium components is also taken into consideration.

The influence of near-resonance self-action of a transversely limited light beam on the integral transmission spectrum of an optically thick saturable absorber with Doppler-broadened transition line is studied numerically for different values of Lorenz-to-Doppler linewidth ratio. It is shown that Doppler broadening diminishes the effect of induced lens and relevant transmission line asymmetry.

In Pasadena, (Milliken Lab., USA, 1930) F. Rossetti has observed in Raman spectrum of carbon-dioxide molecule the full symmetric vibration of carbon dioxide appeared as the group of four near lying lines instead of the waited single line. The true interpretation of this enigmatic effect (in that time) was given by E. Fermi -- accidental degeneration of the first excited state of the full symmetric vibration in carbon dioxide. It was the first example of the event observed later in various organic molecules. This event was named as resonance Fermi. The rotational dependence of Fermi type resonance interactions in quasirigid molecules in dominant approximation can be selected in an expansion of the effective vibration-rotation Hamiltonian H_{vib- rot}^eff by the operator H^(g)(Fermi) equals H₃₀ plus (Sigma) _nH_3n^(g). Let us consider in detail the problem of the construction of the effective vibration-rotational Hamiltonian H_VR yields H^eff from the point of view of various ordering schemes (grouping) of the vibrational-rotational interactions with sequential analysis of the choice of the convenient grouping adequate to the spectroscopic problem.

An effective Hamiltonian describing globally all vibrational-rotational energy levels of C₂H₂ molecule in the ground electronic state has been written up to fourth order in Amat-Nielsen ordering scheme using symmetry arguments. This Hamiltonian contains all resonance interaction terms within polyad scheme discussed by M. E. Kellman [J. Chem. Phys. 95, 8671 (1991)]. D. M. Jonas et al [J. Chem. Phys. 99, 7350 (1993)]. M. Abbouti Temsamani and M. Herman [J. Chem. Phys. 103, 5931 (1995)]. The reduction of this effective Hamiltonian by means of unitary transformations has been performed.

The determination of molecular adiabatic potentials from spectroscopic experimental data is one of the leading problems in the theoretical molecular spectroscopy. The mostly elaborated methods to analyze structured continuums of diatomic molecules with this aim were developed in (repulsive potential) and in (attractive potential). They are based on the uniform harmonic approximation and applicable to those systems only, where the repulsive potential is steeper than the attractive one. New procedures for determining a repulsive or an attractive potential function and a function of the electronic transition moment operator from structured bound-free transition intensity data are described in this paper. These procedures are free from the restrictions of other known approaches to the problems under consideration and can be applied to any structured continuum.

Free CH₂ equals SiH₂ (1) has been observed in the gas phase by millimeter wave spectroscopy. A total of 142 rotational lines of the ²⁸Si species were measured, the search being guided by recent ab initio calculations. The identification of the molecule has been confirmed by the observation of many isotopomers: H₂CSiD₂, H₂CSiHD, H₂C²⁹SiD₂ and D₂CSiH₂. The ab initio and experimental rotational constants of 1 with a planar C_2v structure agree within about 0.5%. Silene was produced by vacuum pyrolysis of Ar-seeded 5,6-bis (trifluoromethyl)-2-silabicyclo [2.2.2] octa-5,7-diene; it decomposes with a 1/e lifetime of the exponential decay at ambient temperature of 30 plus or minus 2 ms.

Between 70 and 500 GHz number of transitions of ⁷⁶SeO₂, ⁷⁷SeO₂, ⁷⁸SeO₂, ⁸⁰SeO₂, and ⁸²SeO₂ in the ground and excited v₂ equals 1 vibrational states have been measured. Transitions of ⁸⁰SeO₂ in the excited v₂ equals 2 state and of ⁷⁴SeO₂ in the ground state have been measured as well. Greatly improved sets of the parameters of A-reduced Watson's Hamiltonian have been obtained in the I^r representation.

The fluorescence excitation and dispersed fluorescence spectra of pyrene and acenaphthene molecules cooled in a supersonic jet have been obtained. Based on the K. Ohno MO/8 model, a new method for calculating frequencies of the in- plane vibrations in the excited electronic states of polycyclic aromatic hydrocarbons is developed. It is shown, that for investigated molecules the prediction accuracy of the vibration frequencies in the ground and first excited electronic states is about 20 cm^-1 without modification of initial parameters, used in the MO/8 model.

Absorption coefficients of pure carbon dioxide and CO₂- He mixture have been measured in the Q-branch of the (20 degree(s)0)III $IMP 01¹0 vibration-rotation band (1880.994 cm^-1) at different pressures from 0.2 to 2.7 atm and temperatures: 294, 314, 333, and 353 K. The experiments were carried out by means of a 1-m multi-pass White cell with an optical path up to 90 m. The tunable CO- laser operated at P20, 8 $IMP 7 (1880.9012 cm^-1) transition was used as a radiation source. The absorption coefficients measured significantly exceeded the results of calculations with the Lorentzian shape due to important effects of line mixing. The temperature dependence of the difference between measured and calculated absorption coefficients are presented.

Measurements are carried out by electron spectroscopy method (ESM) using 127 degree electron spectrometer in crossed electronic and molecular beams. The experimental data of energetical dependences of differential cross sections (EDDCS) of vibrational levels from v equals 0 to v equals 6 for X¹(Sigma) _g⁺-ground state of nitrogen molecule are presented. This research is made at scattering angle 90 degree in (1.4 - 4) eV energy region. Angular and energy resolutions are 2.9 degrees and 50 meV. All EDDCSs involve a vibrational structure at 1.65 - 4 eV. The energetical dependence of resonance structures, obtained from the experimental data, is considered.

Infrared molecular beam depletion spectroscopy has been used to study the absorption of (HF)_n polymers (n equals 2 - 8) embedded in large liquid helium host clusters consisting of a few thousand helium atoms. In the vicinity of 3597 cm^-1 a new absorption band was observed for the first time. The corresponding spectrum is shown in figure 1a. Comparison with the spectra obtained earlier for free gas phase HF clusters reveals that the new feature cannot be assigned to a pure HF cluster. Taking into account that the depletion spectrum was recorded with the mass spectrometer tuned to mass m equals 41 amu, one could assume that it was caused by a hydrogen-bonded heterocomplex containing at least two HF molecules. Since water molecules are easily picked up from the residual gas by the helium clusters it could very well be that the absorption in question is caused by one of the stretching vibrations of the (HF)₂H₂O heterotrimer. To verify this assumption, a theoretical modeling of the vibrational spectra of the (HF)₂H₂O complex was performed for both the free gas phase and the case when it is embedded in a cryogenic Ar matrix. For the latter case accurate experimental data are available. The purpose of the present study was to obtain information on the structure of the heterotrimer and to identify the stretching vibration responsible for the absorption at 3597 cm^-1.

In this paper the results on high resolution spontaneous Raman spectroscopy of barium nitrate molecular ionic crystal at temperature from 8 to 600 K are presented. The temperature dependencies of the linewidth broadening and frequency shifts of internal Raman modes are investigated. The proposed mechanisms of vibronic relaxation of internal Raman modes exhibit low degree of phonon-phonon coupling in the medium. The phonon frequency shifts are found to be due to lattice expansion and are discussed in terms of Gruneisen parameter theory.

We report a registration of magnetic predissociation (MPD) of electronically excited molecules via peculiarities in magnetic field B induced alignment-orientation conversion (AOC). Non-linear magnetic energy shift and heterogeneous MPD produce dispersion type fluorescence circularity signals C(B) of different sign. Measurements on B 1_u^-, v(J) equals 2(96) state of ¹³⁰Te₂ yielded natural C_v^het equals plus or minus 6 s^-1/2 and magnetic (alpha) _v^het equals minus or plus 9 multiplied by 10³ s^-1/2T^-1 rate constants of heterogeneous PD, supposing that the B1_u^- state PD takes place through O_u^- state continuum.

We report here (Lambda) -doubling splitting and permanent electric dipole moment d_p measurements for a number of vibrotational levels of NaK D ¹II state. Two different methods, which are not Doppler limited, were used. Stark effect induced level crossing was registered as fluorescence polarization changes with external electric field, which allowed us to obtain, from one fit, the values of electric dipole moment and (Lambda) -doubling splitting (Delta) _ef between e, f substates of an individual rotational state. Another method consisted in obtaining the ratio (Delta) _ef/d_p from electric field dependence of the intensity of forbidden line appeared in fluorescence as a result of e- f Stark mixing, along with direct (Delta) _ef measurements by optical-rf double resonance. Signal simulations and data fitting were accomplished using direct Hamiltonian diagonalization accounting for Stark interaction within rotational states J plus or minus (Delta) J, (Delta) J equals 0, 1, 2 in initial, excited and final state. Dipole moment values obtained confirm theoretical predictions.

This paper contains the analysis of the A¹(Sigma) _u⁺ of Na₂ based on the data obtained from the polarization labeling spectroscopy experiment on the A¹(Sigma) _u⁺ minus X¹(Sigma) _g⁺ transition. A set of Dunham coefficients is derived, which describes the A state in the wide range of v (0 less than or equal to v less than or equal to 43) and J (12 less than or equal to J less than or equal to 126) quantum numbers and reproduces the positions of unperturbed rotational lines in the A-X band system to within 0.1 cm^-1.

New technique for measurement of the water vapor absorption coefficient for pulse UV radiation is described. This technique allows the useful signal caused by gas absorption to be separated from the signal caused by absorption by walls and windows of the cell. Both the photoacoustic and photoionization signals are observed in the cell by the action of radiation at the wavelength 266 nm upon water vapor and its binary mixtures with the other gases.

Dependence of displaying of lyotropic nematic polymorphism in binary systems tetranuclear palladium organyl/linear alkane from the ratio of number of carbon atoms of aliphatic mesogene substituents and alkane molecule was shown by the method of polmicroscopic analysis of contact preparates. For the first time it has been shown that two phases in nematic region could be resolved not only by polmicroscopic investigation, but also by far infrared Fourier transform (FTIR) spectroscopy.

For the first time far infrared Fourier transform spectroscopy was used for characterization of primary processes of interaction between low level infrared laser radiation and biological systems. It was found that laser radiation influences directly on important organism system compounds (immune, coagulative, anticoagulative, etc.), and laser radiation changes medium condition where these compounds are.

At the first time Fourier transform far infrared spectroscopy in sequences with infrared spectroscopy and viscosimetry was applied to research of new water soluble oil composition possessing higher antiwear resistance, and antirust properties than produced by industry ones. A new structure for these compounds like micelleformation or hexagonal mesogenes without optical anisotropy at 55 - 65 degrees Celsius and defined composition have been found. Obtained data are discussed on base of modern advantages in the field of intermolecular interaction.

A 4.5 micrometer tunable-diode laser based molecular absorption spectrometer is used to investigate the rovibrational level populations in a fast-flowing He/N₂/CO₂ gas discharge. The variation of the rovibrational temperatures is determined over a wide range of discharge parameters relevant for laser action. Good agreement is obtained with a previously developed fully three-dimensional model of the gas flow and vibrational kinetics in non-lasing conditions. The main assumption of the three-temperature model, the near-coincidence of carbon dioxide bending mode temperature and rotational temperature is shown to hold true. In a next step the radial distribution of the vibrational energy is investigated for two different gas inlet sections. The inadequacy of visual observation of the side-light emission is demonstrated and design considerations are formulated. Finally the rovibrational temperatures are measured under lasing conditions, for two different output coupler reflectivities. A maximum electro-optic efficiency of 26% is reached at a carbon dioxide asymmetrical stretch mode temperature reduced to 900 K. Preliminary tests using the same spectrometer of a novel catalyst for CO recombination are reported and an efficiency of 21% is demonstrated under sealed-off operating conditions.

The water vapor absorption spectra between 11,600 and 12,750 cm^-1 have been recorded with the Fourier-transform spectrometer (Kitt-Peak, Az) at the resolution of 0.012 cm^-1 and with the path length of 434 m. Three spectra were used for the analysis: two spectra of water vapor recorded with the Kitt Peak interferometer at the pressure of 1.5 and 17 Torr and the spectrum published by Toth. The experimental details and data reduction procedure have already been discussed, the line assignments were performed simultaneously with calculation of the line positions and intensities that allowed one to obtain an accurate set of experimental rotational energy levels for the eight interacting vibrational states. (131), (211), (013), (230), (112), (032), (051) and (310).

The H₂¹⁸O spectrum has been recently investigated between 11,300 - 13,600 cm^-1 covering the 3v plus (delta) spectral region where v is the quantum of the stretching vibration and (delta) is the quantum of the bending vibration. We present here the study of the 4v and 4v plus (delta) region between 13,000 and 15,500 cm^-1. Spectra of ¹⁸O enriched water vapor have been recorded by means of Fourier-transform spectroscopy. The experimental details have been discussed. The experimental conditions are given.

The Xe₂^*[O_u⁺(³P₁),1_u(³P₂)] excimer emission spectra in non- equilibrium conditions of the low pressure plasma discharge in pure Xe and in the X-He mixture have been measured. The process of vibrational relaxation has been studied and the non-equilibrium vibrational distributions have been obtained.

Spectra of water vapor and carbon dioxide isotopomeres were recorded using ICL-spectrometers at high temperature up to 1350K. The transitions beginning the rotational levels with high J-quantum numbers as well as hot bands were recorded.

A dual-channel photo-acoustic (PA) spectrometer using a single mode 1 W Ti-Sapphire laser with 50 kHz linewidth is described. The value of minimal detectable absorption coefficient equals of 10^-8cm^-1 was determined from the measurements of water absorption lines with well-known intensities. The PA spectrometer was applied to record weak absorption spectra of water, HDO, SiH₄, H₂Se, AsH₃ in 11,300 - 12,800 cm^-1 region. The photo-acoustic (PA) method based on the measurements of an optical radiation absorption by the substance under study. When the pulsed optical radiation passes through the cell containing an absorbing substance (molecular gas in our case) the last one heats due to the absorption. The heat of the gas in a closed volume of the PA cell produces the pulse of the pressure (Delta) p, which was detected by a sensitive microphone proportional optical radiation energy W and absorption coefficient of gas k(v).

The shape of the vibration-rotation v(HF) band is studied in HF/Xe and HF/CO mixtures at increasing Xe (or CO) density from gaseous to liquid phase at 293 K. The evolution of the v(HF) band is determined by coexistence of ensembles of free and complexed molecules. The band of free HF molecules is calculated using the super-Lorentzian profile for rotational HF lines. The v_HF bands of the Xe. . . HF, OC. . .HF and CO. . . HF complexes, which appear in the absorption region of P(1) and P(1)-P(4) rotational lines of HF, are formed by superposition of the vibration-rotation bands due to hot transitions from excited levels of the low-frequency intermolecular modes. The v(subscript HF bands in the complexes experience a low-frequency shift and become more symmetric with increasing density. A model taking into account spectral exchange between overlapping lines provides correct description of the complex band shape evolution with increasing density.

We report an experimental study of the band shapes and relative IR intensities for the stretching v₁ and librational v_L modes of the HF subunit in complexes with OC and HCN. The ratio of the transition dipole moments for the v₁ and v_L bands is found to be close to unity. The relative intensities of the stretching and librational modes in typical H-bonded species (CH₃)₂CO. . .HF and (CH₃)₂O. . .HF are also measured for comparison. A multidimensional quantum mechanical problem of the relative motion of monomers in complexes is solved using model intermolecular interaction potentials and the wave functions expanded in products of 1D-basis functions. Classification of the states of relative motions of monomers is made by considering various cuts through the wave functions. Motions involving different degrees of freedom are shown to be strongly coupled. The frequencies and transition probabilities of the low-frequency modes are computed. The vibrationally averaged electro-optic parameters of complexes evaluated in the framework of the electrostatic induction model are in satisfactory agreement with the experimental data.

The first- and second-order perturbation expressions for the imaginary part of the impact optical cross section for light linear tops at high Js are derived. In this range, the static terms due to V - R coupling are found to be predominant among the contributions to the vibrotational line shift. This opens a new way to deduce data on the vibrational dependence of the intermolecular potential from the experimental shifts of the fundamental and overtone lines. The theory is applied to HCl and HF perturbed by noble-gases and nitrogen. The derived overdetermined system of equations for the characteristics of the intermolecular potential is processed by the linear regression analysis. The results thus obtained are compared with the available potential models.

In order to investigate the temperature dependence of broadening parameters and band profiles we have studied the v₂ and 3v₃ bands of carbon-dioxide perturbed by He at 193 K. We used the energy corrected sudden (ECS) scaling law associated with the power fitting law. Experiments were carried out with a Fourier transform spectrometer.

The computational estimations of the contributions of polarization interactions into the shifts were made. Such analysis allows one to define the value of contributions from the first and second orders of perturbation theory into the shifts and to solve the inverse problem for hyper- polarizability. Analysis for the shifts of H₂O, S₂O, NO₂, NO, H₂S, O₃, CO polar molecule absorption lines induced by Ar pressure were carried out. Estimation results shows the contribution of high order polarization interaction can reach 35%.

It is well known that infrared inactive gases, such as nitrogen and hydrogen, absorb infrared radiation if the gas densities are sufficiently high. The absorption is of a supramolecular nature, involving dipole moments induced by exchange, overlap, and dispersion forces as well as by multipole induction in pairs, triples, . . . of interacting molecules. The same collision-induced dipoles also emit radiation in the infrared and, at sufficiently high temperatures, also in the visible and near ultraviolet. We have estimated the intensities and the spectral profiles of binary emission spectra of nitrogen and nitrogen-argon mixtures for conditions believed to be representative for the converging spherical shockwaves of sonoluminescence, e.g., for densities of several times 10²¹ molecules per cubic centimeters and temperatures of tens to hundreds of thousands of kelvin. Remarkably, the calculated profiles and intensities reproduce the measurements of such spectra in detail in the spectral range from 200 to 700 nm, the window of water, where the spectra are known. The sonoluminescence source is shown to be optically thin.

In order to study the influence of correlation of the angular momenta before and after collision on the course of orientational relaxation of spherical top molecules, a Markovian master equation with the collision kernel by Keilson and Storer is considered. The equation is solved analytically in the case of the complete angular momentum reorientation due to a collision. The results obtained allow one to establish the upper and the lower limit of the time development of the orientational correlation functions.